you can try the stern backglass top from pinlife here:
http://www.pinballlife.com/index.php?p=catalog&parent=213&pg=1 (pretty sure you'll need 2 per marquee if you do both the top and bottom).
or
you can use poster edging, which is sold at crafts stores/wallyworld/most places that sell posters (it's a thin plastic channel that goes on the top and bottom of posters, it's a pain to work with but it works).
If you get the stuff from pinlife let me know what you think of it as i was planning on using it myself.
you only really need it for the top of the marquee not so much the bottom.